Proton conductor

ABSTRACT

An exemplary proton conductor according to the present disclosure has a perovskite-type crystal structure expressed by the compositional formula A a B 1−x B′ x O 3−δ , where A is at least one selected from among group 2 elements; B is a group 4 element or Ce; B′ is a group 3 element, a group 13 element, or a lanthanoid element; 0.5&lt;a≦1.0, 0.0≦x≦0.5, and 0.0≦δ&lt;3; and the charge of the above compositional formula is deviated from electrical neutrality in a range of −0.13 or more but less than +0.14.

This application is a Divisional of U.S. patent application Ser. No.14/469,165, filed on Aug. 26, 2014, which is a continuation ofInternational Application No. PCT/JP2014/000515, filed on Jan. 31, 2014,the contents of which are hereby incorporated by reference.

BACKGROUND

1. Technical Field

The present disclosure relates to a proton conductor. Moreover, thepresent disclosure relates to various devices having a proton conductor.

2. Description of the Related Art

Among proton conducting solid electrolytes, many perovskite-type protonconductors have been reported which are expressed by the compositionalformula AB_(1−x)B′_(x)O_(3−δ). Herein, A is an alkaline-earth metal; Bis a tetravalent group 4 transition metal element, or Ce, which is atetravalent lanthanoid element; B′ is a trivalent group 3 or group 13element; and O is oxygen. x is a mole fraction of the B′ element withwhich the B element is substituted, satisfying 0<x<1.0. δ is a valuerepresenting oxygen deficiencies or oxygen excesses. The fundamentalconstruction of a perovskite structure will later be briefly describedwith reference to the drawings.

Nature materials Vol 9 (October 2010) 846-852 discloses oxides of aperovskite structure. The oxides described in Nature materials Vol 9(October 2010) 846-852 have the compositional formulaBaZr_(1−x)Y_(x)O_(3−δ) or the compositional formulaBaCe_(1−x)Y_(x)O_(3−δ). In these oxides, A is barium (Ba); B is Zr orCe; and B′ is Y.

Japanese Laid-Open Patent Publication No. 2008-23404 discloses a protonconducting film of a perovskite structure. The proton conducting filmdescribed in Japanese Laid-Open Patent Publication No. 2008-23404 hasthe chemical formula AL_(1−X)M_(X)O_(3−δ). A is an alkaline-earth metal.L is one or more kinds of elements selected from cerium, titanium,zirconium, and hafnium. M is one or more kinds of elements selected fromneodymium, gallium, aluminum, yttrium, indium, ytterbium, scandium,gadolinium, samarium, and praseodymium. Herein, X is the mole fractionof an M element with which the L element is substituted, where α is anatomic ratio of oxygen deficiencies. In the proton conducting filmdescribed in Japanese Laid-Open Patent Publication No. 2008-23404,0.05<X<0.35, and 0.15<α<1.00.

SUMMARY

One non-limiting, illustrative embodiment of present disclosure providesa perovskite-type proton conductor which has high proton conductivityeven in a temperature region of not less than 100° C. and not more than500° C.

In one general aspect, a proton conductor disclosed herein has aperovskite-type crystal structure expressed by the compositional formulaA_(a)B_(1−x)B′_(x)O_(3−δ), where A is at least one selected from amonggroup 2 elements; B is at least one selected from among group 4 elementsand Ce; B′ is a group 3 element, a group 13 element, or a lanthanoidelement; 0.5<a≦1.0, 0.0≦x≦0.5, and 0.0≦δ<3; and charge of thecompositional formula is deviated from electrical neutrality in a rangeof −0.13 or more but less than +0.14.

According to a non-limiting, illustrative embodiment of presentdisclosure, a perovskite-type proton conductor having high protonconductivity can be provided.

Additional benefits and advantages of the disclosed embodiments will beapparent from the specification and Figures. The benefits and/oradvantages may be individually provided by the various embodiments andfeatures of the specification and drawings disclosure, and need not allbe provided in order to obtain one or more of the same.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a generic perovskite structure expressed bythe compositional formula ABO₃.

FIG. 2 is a diagram showing proton conductivity in a temperature rangefrom 100° C. to 600° C. according to Example 1.

FIG. 3 is a cross-sectional view showing an exemplary device including aproton conductor.

DETAILED DESCRIPTION Perovskite Structure

As illustrated in FIG. 1, the generic perovskite structure is composedof elements A, B, and O, and expressed by the compositional formulaABO₃. Herein, A is an element which may become a divalent cation; B isan element which may become a tetravalent cation; and O is oxygen. Theunit lattice of a crystal having a perovskite structure typically has anear cubic shape. As shown in the figure, ions of element A are locatedon the eight vertices of the unit lattice. On the other hand, ions ofoxygen O are located at the centers of the six faces of the unitlattice. Moreover, an ion of element B is located near the center of theunit lattice. The positions occupied by elements A, B, and O may becalled the A site, the B site, and the O site, respectively.

The above structure is the basic structure of a perovskite crystal, inwhich some of elements A, B, and O may be deficient, excessive, orsubstituted by other elements. For example, a crystal in which elementB′ other than element B is located at the B site is a perovskite crystalwhich is expressed by the compositional formula AB_((1−x))B′_(x)O₃.Herein, x is a mole fraction (ratio of the number of atoms) of B′, whichmay be referred to as the substitution ratio. When such substitution,deficiency, or excess of elements occurs, the structure of the unitlattice may distorted or deformed from being a cube. The perovskitecrystal is not limited to “cubic”, but broadly encompasses any crystalwhich has undergone a phase transition into the less-symmetric “rhombic”or “tetragonal”.

(Findings of the Inventors)

In a conventional proton conductor having a perovskite structure,substituting a tetravalent element B with a trivalent element B′ causesoxygen deficiencies in the proton conductor. This is considered because,when some of the tetravalent cations are substituted with trivalentcations, the total positive charge possessed by the cations decreases sothat the mole fraction of oxygen ions, which are divalent anions,decreases due to a charge compensation action towards maintainingelectrical neutrality, thereby causing oxygen deficiencies. In a protonconductor having such a composition, it is considered that carriers ofproton conduction are introduced into the proton conductor as watermolecules (H₂O) are introduced at the positions (O sites) of oxygendeficiencies.

Conventional proton conductor are considered to exhibit protonconductivity because protons undergo hopping conduction around theoxygen atoms. In this case, temperature dependence of protonconductivity manifests itself in a thermal-activation profile, with anactivation energy on the order of 0.4 to 1.0 eV. Therefore, protonconductivity undergoes an exponential decrease with decreasingtemperature.

In order for the proton conductor to maintain a high proton conductivityof 10⁻² S/cm (Siemens/centimeter) or more even in the temperature regionof not less than 100° C. and not more than 500° C., it is beneficial toensure that the activation energy concerning proton conductivity is 0.1eV or less, thereby suppressing any decrease in proton conductivity thatis caused by decreasing temperature.

The inventors have tried to create a situation where protons can movemore easily than via conventional hopping by increasing the solidsolution amount (amount of substitution) of the trivalent element B′ soas to increase the concentration or density of proton carriers. However,in a conventional perovskite-type proton conductor, the upper limit ofthe mole fraction of the B′ element is about 0.2, which presents anupper limit to the amount of oxygen deficiencies.

As a method of introducing more proton carriers, the inventors havefound that similar effects to increasing the mole fraction of the B′element can be obtained by decreasing the mole fraction of the Aelement. This allows, as described earlier, more oxygen-deficient sitesthan conventionally to be provided in the proton conductor. It was alsofound that the concentration or density of proton carriers can beenhanced, furthermore, by introducing water molecules at the positionsof oxygen deficiencies, or introducing protons near oxygens so that thecharge of the compositional formula of the perovskite structure isdeviated from electrical neutrality. As a result of this, aperovskite-type proton conductor having high proton conductivity wasobtained. One implementation of the present disclosure is as follows, inoutline.

An proton conductor according to one implementation of the presentdisclosure is a proton conductor having a perovskite-type crystalstructure expressed by the compositional formulaA_(a)B_(1−x)B′_(x)O_(3−δ), where A is at least one selected from amonggroup 2 elements; B is at least one selected from among group 4 elementsand Ce; B′ is a group 3 element, a group 13 element, or a lanthanoidelement; 0.5<a≦1.0, 0.0≦x≦0.5, and 0.0≦δ<3; and charge of thecompositional formula is deviated from electrical neutrality in a rangeof −0.13 or more but less than +0.14.

A may be at least one selected from the group consisting of Ba, Sr, andCa; B may be at least one selected from the group consisting of Zr, Ce,and Ti; and B′ may be one selected from the group consisting of Yb, Y,Nd, and In.

It may be that x=0, and the charge of the above compositional formulamay be deviated from electrical neutrality in a range of more than 0 butless than 0.14.

It may be that 0.0<x≦0.50, and the charge of the above compositionalformula may be deviated from electrical neutrality in a range of −0.13or more but less than 0.

In one embodiment, the values a, x and δ are determined based onquantitative measurements of elements composing the proton conductor.

In one embodiment, an activation energy of proton conduction in atemperature range of not less than 100° C. and not more than 500° C. is0.1 eV or less.

Embodiment 1

Hereinafter, embodiments will be described.

The proton conductor of the present disclosure has a perovskite-typecrystal structure expressed by the compositional formulaA_(a)B_(1−x)B′_(x)O_(3−δ). A is at least one selected from amongalkaline-earth metals. B is at least one selected from among group 4transition metals and Ce. B′ is a group 3 or group 13 element. In theabove compositional formula, a, x, and δ satisfy 0.5<a≦1.0, 0.0≦x≦0.5,and 0.0≦δ<3. Moreover, the charge of the above compositional formula isdeviated from electrical neutrality in a range of −0.13 or more but lessthan +0.14. In other words, the mole fractions a and x, and amount ofoxygen deficiencies δ are determined so that the compositional formulaA_(a)B_(1−x)B′_(x)O_(3−δ) is not electrically neutral. The crystalcomposition of the proton conductor itself is electrically neutral; itis considered that, in a manner of compensating the charge of theperovskite-type crystal structure expressed by the compositional formulaA_(a)B_(1−x)B′_(x)O_(3−δ), the perovskite-type crystal structure mayhave protons introduced therein, or an electrical excess of oxygen.

In the present specification, the crystal composition of the protonconductor includes the proton conductor as well as protons, oxygen, orthe like that compensate the charge of the perovskite-type crystalstructure. Note that the protons, oxygen, or the like that compensatethe charge of the perovskite-type crystal structure are located inaccordance with the composition, charge condition, etc., of the protonconductor, these protons or oxygen not having been intentionallyintroduced. Therefore, the proton conductivity of the proton conductoris considered to mainly depend on the composition and charge conditionof the proton conductor.

<A Element>

Examples of the A element are group 2 elements (alkaline-earth metals).The A element being a group 2 element stabilizes the perovskite-typestructure. Typical examples of the A element are at least one selectedfrom the group consisting of barium (Ba), strontium (Sr), calcium (Ca),and magnesium (Mg). In particular, those proton conductors whose Aelement is at least one selected from the group consisting of calcium(Ca), barium (Ba), and strontium (Sr) can have high proton conductivity.Moreover, the A element may at least contain barium (Ba) andadditionally contain at least one selected from the group consisting ofstrontium (Sr), calcium (Ca), and magnesium (Mg). For example, the Aelement is Ba_(y)A′_(1−y)(0<y≦1).

Since the A element is a group 2 element whose valence is divalent,similar effects to increasing the mole fraction of the B′ element amountcan be obtained by decreasing the amount of the A element, thus makingoxygen deficiencies likely to occur. This causes a deviation fromelectrical neutrality, making it easier for proton carriers to beintroduced; thus, an effect of enhancing the proton carrierconcentration is obtained.

<B Element>

Examples of the B element are at least one selected from group 4elements and cerium (Ce). Typical examples of the B element are at leastone selected from the group consisting of zirconium (Zr), cerium (Ce),titanium (Ti), and hafnium (Hf). When the B element is zirconium (Zr),the perovskite-type structure will be stable, thus resulting in lessproduction of any structural components not possessing protonconductivity. As a result, proton conductor having high protonconductivity can be obtained.

<B′ Element>

Examples of the B′ element are group 3 elements, group 13 elements, andtrivalent lanthanoid elements. For example, the B′ element may have anion radius greater than 0.5 Å and smaller than 1.02 Å. This allows tokeep the perovskite-type structure stable. Thus, while maintaining thecrystal structure, oxygen deficiencies are likely to occur, thuspermitting stable existence even if the charge of the abovecompositional formula A_(a)B_(1−x)B′_(x)O_(3−δ) is deviated fromelectrical neutrality.

Typical examples of the B′ element are ytterbium (Yb), neodymium (Nd),yttrium (Y), and indium (In). It is more beneficial that a protonconductor whose B′ element is ytterbium (Yb), neodymium (Nd), yttrium(Y), or indium (In) because its perovskite structure is stable and italso has a high proton conductivity.

(a, x, and δ)

The value a, which represents the mole fraction of the A element, is inthe range of 0.5<a≦1.0. When the mole fraction a is 0.5 or less, it ispossible to synthesize an oxide having a perovskite structure, but it isdifficult to control the charge of the above compositional formula fromelectrical neutrality, this being not preferable.

When the mole fraction a is greater than 1.0, the oxide will have aphase not exhibiting proton conductivity, thus resulting in a greatlyreduced proton conductivity.

The x value, which represents the mole fraction of the B′ element, is inthe range of 0.0≦x≦0.5. The mole fraction x being greater than 0 willcause oxygen deficiencies in the perovskite structure, so that carriersof proton conduction will be introduced into the proton conductor. Onthe other hand, when the mole fraction x is greater than 0.5, it ispossible to synthesize an oxide having a perovskite structure, but aphase not exhibiting proton conductivity will also be produced. Thiswill greatly reduce the proton conductivity of the entire oxide.

In the above compositional formula, A is a divalent element; B is atetravalent element; B′ is a trivalent element; and oxygen is a divalentelement. Therefore, the charge of the above compositional formula isdetermined by the mole fractions a and x and the amount of oxygendeficiencies δ. In terms of electrical neutrality, it is considered thata sum of the amount of A deficiencies, i.e., the value (1−a), and a halfamount of the amount of B′ substitution defines the amount of oxygendeficiencies. In the proton conductor of the present embodiment, thecharge of the above compositional formula is deviated from electricalneutrality in a range of −0.13 or more but less than +0.14. In otherwords, the mole fractions a and x, and amount of oxygen deficiencies δare determined so that the compositional formulaA_(a)B_(1−x)B′_(x)O_(3−δ) is not electrically neutral.

For example, when the mole fraction x of B′ is 0, the charge of theabove compositional formula is deviated from electrical neutrality in arange of more than 0 but less than 0.14. When the mole fraction x of B′is such that 0.0<x≦0.5, the charge of the above compositional formula isdeviated from electrical neutrality in a range of −0.13 or more but lessthan 0.

When the mole fractions a and x are such that 0.5<a≦1.0 and 0.0≦x≦0.5, astable oxide having plenty of oxygen deficiencies and having aperovskite structure is obtained. As a result of water molecules orprotons being introduced at the positions of these oxygen deficiencies,or the positions of oxygen deficiencies being left vacant, the charge ofthe above compositional formula is deviated from electrical neutralityin a range of −0.13 or more but less than +0.14. Although the detailedreasons are currently unclear, this presumably realizes a protonconductor which has a high proton carrier concentration, or in whichprotons are likely to conduct at relatively low temperatures in terms ofcharge distribution in the crystal structure. According to the presentembodiment, a proton conductor having a proton conductivity of about10⁻¹ S/cm at a temperature of about 100° C. can be realized.

According to the present disclosure, a proton conductor is realizedwhich has a single-crystalline or polycrystalline perovskite structurecomposed of a single phase that is substantially uniform (homogeneous)in composition and crystal structure. Herein, being “composed of asingle phase which is substantially uniform in composition and crystalstructure” means that the proton conductor does not contain anyheterophase that has a composition outside the ranges of the presentinvention. Note that embodiments of the proton conductor of the presentdisclosure may contain minute amounts of unavoidable impurities. In thecase where the proton conductor of the present disclosure is produced bysintering, compounds or elements of sintering aids or the like may bepartially contained. Otherwise, in the course of the production process,impurities may be added unintentionally, or intentionally for certaineffects. What is important is that the respective elements of A, B, B′,and O are within the ranges defined by the present disclosure, theseconstituting a perovskite crystal structure. Therefore, impurities whichmight stray in during production may be contained.

(Determination of Compositional Formula and Measurement of Charge of theCompositional Formula)

The compositional formula of the proton conductor of the presentembodiment, and a charge deviation from electrical neutrality when it isexpressed by the above compositional formula, can be measured by usingan electron probe microanalyzer (EPMA), for example. For example,quantitative measurements of elements of the proton conductor asrepresented by the above compositional formula are taken, and the chargeof the above compositional formula is calculated from the valences andmole fractions of the component elements. In an evaluation using theelectron probe microanalyzer used for measurement, it is beneficial totake measurements by using a spectroscope of a wavelength dispersiontype. In this case, it is beneficial to conduct calibration by using acontrol sample having a known chemical composition, so that quantitativeevaluation is possible. On the other hand, a spectroscope of an energydispersion type is not preferable because it is prone to large errors inquantitative evaluation of light elements such as oxygen. Moreover, aquantitative analysis by using inductively coupled plasma spectroscopy(ICP) may be able to measure metallic elements composing the material,but not able to quantify the oxygen element; thus, it is not suitablefor measuring electrical neutrality deviations.

As the electron probe microanalyzer (EPMA), JXA-8900R manufactured byJEOL Ltd. can be used, for example. For a characteristic X ray whichoccurs through irradiation of an electron beam, a wavelength dispersivespectroscope (WDS) is used. The characteristic X ray is measured throughquantitative measurement, by using a characteristic X ray that has beenobtained through calibration with a control sample having a knowncomposition, and using an analyzing crystal. The component element ratioof the proton conductor is analyzed, and a product of the valence andthe component element fraction of each component element is calculated.For example, in the perovskite-type crystal structure expressed by thecompositional formula A_(a)B_(1−x)B′_(x)O_(3−δ), let it be assumed thatA is Ba; B is Zr; and B′ is Y. In this case, calculations are to beperformed by assuming that Ba has +2 valence; Zr has +4 valence; Y has+3 valence; and O has −2 valence. As the valence of any such element,the most likely valence value in a stable oxide state is to be adopted.Next, assuming that an analysis using EPMA finds that the fraction ofA(Ba) is a; the fraction of B(Zr) is b; the fraction of B′ (Y) is c; andthe fraction of O is d, then the chemical compositional formula can beexpressed as Ba_(a)Zr_(b)Y_(c)O_(d). The charge of the compositionalformula, i.e., deviation from electrical neutrality, can be calculatedto be ((2×a)+(4×b)+(3×c)−(2×d))÷(b+c). Through this calculation, aproduct of the valence of each component element and its componentelement fraction is taken, and a sum of the values of the respectiveelements is derived, after which the charge is normalized by utilizing asum of the component element fractions of B and B′ as 1. Since B and B′are unlikely to become deficient because of they are located in thecenter of an octahedron which is surrounded by oxygens in theperovskite-type structure, a normalization based on a sum of B and B′ isapplied to ascertain a deviation of charge from electrical neutralityper unit cell (unit lattice).

It is beneficial that the ion conductor to be analyzed has a flatsurface because, if there were significant rises and falls,characteristic X rays occurring from within the sample upon electronbeam irradiation would not be correctly detected, thus makingquantitative measurement difficult. Moreover, since proton conductorshave poor electrical conductivity, carbon or the like may be thinlyvapor-deposited on the surface to prevent charging with an incidentelectron beam. There is no particular limitation as to the incidencecondition of the electron beam for observation, so long as a sufficientsignal intensity is obtained at the WDS, and as the sample is not bedegraded via burning or the like caused by incidence of the electronbeam. As for the measuring points, it is beneficial to conductmeasurement at five or more points and adopt an average value thereof.Among the five or more measuring points, it is beneficial that anymeasurement value which is clearly distant from the other measurementvalues is not adopted as a value for average value calculation.

(Production Method)

The proton conductor of the present embodiment can be implemented invarious forms. In order to obtain a film of proton conductor, it can beproduced by a film formation method such as a sputtering technique, apulsed laser deposition technique (PLD technique), or a chemical vapordeposition technique (CVD technique). Adjustment of the composition isachieved by commonly-used techniques used in such film formationmethods. For example, in the case of a sputtering technique or a pulsedlaser deposition technique, the target composition may be adjusted tocontrol the composition of the proton conductor that is produced. In thecase of a chemical vapor deposition technique, the amount of source gasto be introduced into the reaction chamber may be adjusted to controlthe composition of the proton conductor that is produced.

In order to obtain a proton conductor in bulk state, it can besynthesized by a solid phase reaction technique, a hydrothermalsynthesis method, or the like.

In order to adjust for a charge deviation from electrical neutralitywhen the proton conductor of the present embodiment is expressed by theabove compositional formula, the molar ratio between the componentelements under the aforementioned production methods may be controlled,or a heat treatment in a reducing atmosphere may follow synthesis toachieve control.

(Others)

Proton conductors are also referred to as called proton conducting solidelectrolytes. A proton conductor does not need to be a continuous filmor in bulk form, so long as it functions to conduct protons.

In the case where the proton conductor is implemented in film form, thesurface of the base substrate on which the proton conductor is supporteddoes not need to be flat. In order to prevent direct reaction betweenthe gas which is the source material that supplies protons to the protonconductor of the present embodiment and the gas that reacts with theprotons having been conducted through the proton conductor or thehydrogen which is the reduced form of protons, it is beneficial thatthere is no leakage between the flow paths of these two gases. To thisend, for example, a thin film of the perovskite-type proton conductor ofthe present embodiment may be formed on a base substrate which iscomposed of magnesium oxide (MgO), strontium titanate (SrTiO₃), silicon(Si), or the like, this base substrate having a smooth plane.Thereafter, a part or a whole of the base substrate may be removed byusing etching or the like, thus partially exposing the surface of theproton conductor through the base substrate for supplying gases. Thereis no particular limitation as to the material and shape of the basesubstrate.

The crystal structure of the proton conductor may be single-crystallineor polycrystalline, as described above. A proton conductor havingoriented crystal structure by controlling the orientation of crystalgrowth on a substrate of magnesium oxide (MgO) or strontium titanate(SrTiO₃), or on a silicon (Si) substrate having a buffer layer with acontrolled lattice constant formed thereon, can have a higher protonconductivity. A proton conductor having single-crystalline structurewhich is epitaxially grown on a substrate can have a higher protonconductivity. The proton conductor can acquire a single-crystallinestructure through control of the film-formation conditions such as thesurface orientation of the substrate, temperature, pressure, and theatmosphere, for example. There is no particular limitation as to theconditions for obtaining a single-crystalline structure and the crystalgrowth direction or orientation direction of the single-crystallinestructure.

(Device Applications)

By using the proton conductor of the present disclosure for a knowndevice, a device having high proton conduction can be provided. FIG. 3shows an exemplary device including the proton conductor. The deviceshown in FIG. 3 includes a proton conductor 1, an anode electrode 2, anda cathode electrode 3. Examples of known devices are fuel cells,hydrogen sensors, water vapor electrolytic devices, and hydrogenationdevices.

EXAMPLES

Hereinafter, the present disclosure will be specifically described byway of Examples.

Example 1

A base substrate (10 mm×10 mm, thickness 0.5 mm) was set on a substrateholder within a vacuum chamber, the substrate holder having a heatingmechanism, and the inside of the vacuum chamber was evacuated to adegree of vacuum of about 10⁻³ Pa. The material of the base substratewas single-crystalline magnesium oxide (MgO).

After evacuating the inside of the vacuum chamber, the base substratewas heated at 650° C. to 750° C. An oxygen gas (flow rate: 2 sccm) andan argon gas (flow rate: 8 sccm) were introduced, and the pressureinside the vacuum chamber was adjusted to about 1 Pa.

By using a sintered target having an element ratio of Ba:Zr=1:1, aproton conductor was formed into a film by a sputtering technique. Theresultant proton conductor had a thickness of 500 nm, and a size of 10mm×10 mm.

The structure, mole fractions, and proton conductivity of the resultantfilm of proton conductor were evaluated. Moreover, the charge deviationand the activation energy of proton conduction were determined. Resultsare shown in Table 1. Hereinafter, the respective evaluation methods andthe results thereof will be described.

By using a Cu target, an X-ray diffraction of the proton conductorproduced was measured. It was confirmed that the resultant protonconductor had a perovskite-type crystal structure.

The compositional formula and the charge of the compositional formula ofthe proton conductor produced were measured by the aforementioned EMPA.As shown in Table 1, in the compositional formula of the protonconductor (A_(a)BO_(3−δ)), the A element was barium (Ba) and the a valuewas 0.904 assuming that the B element of zirconium was 1. Moreover, theoxygen amount (3−δ) was 2.876, and the charge of the compositionalformula was deviated from electrical neutrality by 0.056 toward thepositive side.

An electrode was formed by using silver paste on the proton conductor.In an argon (Ar) gas in which 5% hydrogen (H₂) was mixed, under atemperature-range condition from 100° C. to 600° C., proton conductivitywas measured by using an impedance method. Temperature dependence ofproton conductivity is shown in FIG. 2.

As shown in Table 1, the proton conductivity at 100° C. was 0.12 S/cm,and the proton conductivity at 500° C. was 0.25 S/cm. The activationenergy of proton conduction was determined to be 0.041 eV.

Example 2

An experiment was conducted similarly to Example 1 except that a filmwas formed by using a sintered target having an element ratio ofBa:Zr:Ce:Nd=10:5:4:1. Table 1 shows the structure, mole fractions, andproton conductivity of the resultant film of proton conductor.

It was confirmed that the proton conductor had a perovskite-type crystalstructure. As shown in Table 1, in the proton conductor(A_(a)B_(1−x)B′_(x)O_(3−δ)), the A element was barium (Ba); the a valuewas 0.975; the B element was zirconium (Zr) and cerium (Ce), theirrespective values being 0.495 and 0.412. The B′ element was neodymium(Nd), with an x value of 0.099. The oxygen amount (3−δ) was 2.975, andit was found that, given the tetravalence of Ce, the charge of thecompositional formula was deviated from electrical neutrality by 0.099toward the negative side. As shown in Table 1, the proton conductivityat 100° C. was 0.33 S/cm, and the proton conductivity at 500° C. was0.48 S/cm. The activation energy of proton conduction was determined tobe 0.022 eV.

Example 3

An experiment was conducted similarly to Example 1 except that a filmwas formed by using a sintered target having an element ratio ofBa:Zr:Y=2:1:1. Table 1 shows the structure, mole fractions, and protonconductivity of the resultant film of proton conductor.

It was confirmed that the proton conductor had a perovskite-type crystalstructure and was polycrystalline. As shown in Table 1, in the protonconductor (A_(a)B_(1−x)B′_(x)O_(3−δ)), the A element was barium (Ba);the a value was 0.539; the B element was zirconium (Zr); and the B′element was yttrium (Y), with an x value of 0.48 (Zr:0.52, Y:0.48). Theoxygen amount was 2.363, and it was found that, given the tetravalenceof Ce, the charge of the compositional formula was deviated fromelectrical neutrality by 0.128 toward the negative side. As shown inTable 1, the proton conductivity at 100° C. was 0.39 S/cm, and theproton conductivity at 500° C. was 0.71 S/cm. The activation energy ofproton conduction was determined to be 0.035 eV.

Example 4

An experiment was conducted similarly to Example 1 except that a filmwas formed by using a sintered target having an element ratio ofBa:Zr:Ti:In=6:7:1:2. Table 1 shows the structure, mole fractions, andproton conductivity of the resultant film of proton conductor.

It was confirmed that the proton conductor had a perovskite-type crystalstructure. As shown in Table 1, in the proton conductor(A_(a)B_(1−x)B′_(x)O_(3−δ)), the A element was barium (Ba); the a valuewas 0.522; the B element was zirconium (Zr) and titanium (Ti), withzirconium being 0.682 and titanium being 0.098. Moreover, the oxygenamount was 2.474, and it was found that the charge of the compositionalformula was deviated from electrical neutrality by 0.124 toward thenegative side. The B′ element was indium (In), with an x value of 0.22.As shown in Table 1, the proton conductivity at 100° C. was 0.34 S/cm,and the proton conductivity at 500° C. was 0.67 S/cm. The activationenergy of proton conduction was determined to be 0.037 eV.

Example 5

Barium carbonate (BaCO₃), zirconium dioxide (ZrO₂), and yttrium oxide(Y₂O₃) were weighed and mixed, and prebaked at about 1300° C. andthereafter baked at 1700° C., whereby a bulk sample of ceramic wasproduced so as to attain an element ratio of Ba:Zr:Y=5:4:1. Except forusing the bulk body of ceramic, an experiment was conducted similarly toExample 1. Table 1 shows the structure, mole fractions, and protonconductivity of the resultant film of proton conductor.

It was confirmed that the proton conductor had a perovskite-type crystalstructure. As shown in Table 1, in the proton conductor(A_(a)B_(1−x)B′_(x)O_(3−δ)), the A element was barium (Ba); the a valuewas 0.966; the B element was zirconium (Zr); and the B′ element wasyttrium (Y), with an x value of 0.197. Moreover, the oxygen amount was2.932, and it was found that the charge of the compositional formula wasdeviated from electrical neutrality by 0.129 toward the negative side.As shown in Table 1, the proton conductivity at 100° C. was 0.37 S/cm,and the proton conductivity at 500° C. was 0.78 S/cm. The activationenergy of proton conduction was determined to be 0.039 eV.

Example 6

An experiment was conducted similarly to Example 1 except that a filmwas formed by using a sintered target having an element ratio ofSr:Zr=1:1. Table 1 shows the structure, mole fractions, and protonconductivity of the resultant film of proton conductor.

It was confirmed that the proton conductor had a perovskite-type crystalstructure. As shown in Table 1, in the proton conductor (A_(a)BO_(3−δ)),the A element was strontium (Sr), with an a value of 0.98. Moreover, theoxygen amount was 2.969, and it was found that the charge of thecompositional formula was deviated from electrical neutrality by 0.022toward the positive side. As shown in Table 1, the proton conductivityat 100° C. was 0.10 S/cm, and the proton conductivity at 500° C. was0.21 S/cm. The activation energy of proton conduction was determined tobe 0.055 eV.

Example 7

An experiment was conducted similarly to Example 1 except that a filmwas formed by using a sintered target having an element ratio ofSr:Zr:Yb=10:7:3. Table 1 shows the structure, mole fractions, and protonconductivity of the resultant film of proton conductor.

It was confirmed that the proton conductor had a perovskite-type crystalstructure. As shown in Table 1, in the proton conductor(A_(a)B_(1−x)B′_(x)O_(3−δ)), the A element was strontium (Sr), with an avalue of 0.965. The B element was zirconium (Zr); and the B′ element wasytterbium (Yb), with an x value of 0.05. Moreover, the oxygen amount was2.955, and it was found that the charge of the compositional formula wasdeviated from electrical neutrality by 0.03 toward the negative side. Asshown in Table 1, the proton conductivity at 100° C. was 0.10 S/cm, andthe proton conductivity at 500° C. was 0.22 S/cm. The activation energyof proton conduction was determined to be 0.055 eV.

Example 8

An experiment was conducted similarly to Example 1 except that a filmwas formed by using a sintered target having an element ratio ofSr:Zr:Ce:Y=8:4:3:3. Table 1 shows the structure, mole fractions, andproton conductivity of the resultant film of proton conductor.

It was confirmed that the proton conductor had a perovskite-type crystalstructure. As shown in Table 1, in the proton conductor(A_(a)B_(1−x)B′_(x)O_(3−δ)), the A element was strontium (Sr), with an avalue of 0.736. The B element was zirconium (Zr) and cerium (Ce), withzirconium being 0.423 and cerium being 0.307. The B′ element was yttrium(Y), with an x value of 0.27. Moreover, the oxygen amount was 2.616, andit was found that the charge of the compositional formula was deviatedfrom electrical neutrality by 0.03 toward the negative side. As shown inTable 1, the proton conductivity at 100° C. was 0.13 S/cm, and theproton conductivity at 500° C. was 0.25 S/cm. The activation energy ofproton conduction was determined to be 0.042 eV.

Example 9

An experiment was conducted similarly to Example 1 except that a filmwas formed by using a sintered target having an element ratio ofSr:Zr:In=7:6:4. Table 1 shows the structure, mole fractions, and protonconductivity of the resultant film of proton conductor.

It was confirmed that the proton conductor had a perovskite-type crystalstructure. As shown in Table 1, in the proton conductor(A_(a)B_(1−x)B′_(x)O_(3−δ)), the A element was strontium (Sr), with an avalue of 0.686. The B element was zirconium (Zr); and the B′ element wasindium (In), with an x value of 0.351. Moreover, the oxygen amount was2.542, and it was found that the charge of the compositional formula wasdeviated from electrical neutrality by 0.063 toward the negative side.As shown in Table 1, the proton conductivity at 100° C. was 0.13 S/cm,and the proton conductivity at 500° C. was 0.24 S/cm. The activationenergy of proton conduction was determined to be 0.041 eV.

Example 10

An experiment was conducted similarly to Example 1 except that a filmwas formed by using a sintered target having an element ratio ofSr:Zr:Y=3:3:2. Table 1 shows the structure, mole fractions, and protonconductivity of the resultant film of proton conductor.

It was confirmed that the proton conductor had a perovskite-type crystalstructure. As shown in Table 1, in the proton conductor(A_(a)B_(1−x)B′_(x)O_(3−δ)), the A element was strontium (Sr), with an avalue of 0.563. The B element was zirconium (Zr); and the B′ element wasyttrium (Y), with an x value of 0.474. Moreover, the oxygen amount was2.358, and it was found that the charge of the compositional formula wasdeviated from electrical neutrality by 0.063 toward the negative side.As shown in Table 1, the proton conductivity at 100° C. was 0.18 S/cm,and the proton conductivity at 500° C. was 0.30 S/cm. The activationenergy of proton conduction was determined to be 0.025 eV.

Example 11

An experiment was conducted similarly to Example 1 except that a filmwas formed by using a sintered target having an element ratio ofCa:Zr=1:1. Table 1 shows the structure, mole fractions, and protonconductivity of the resultant film of proton conductor.

It was confirmed that the proton conductor had a perovskite-type crystalstructure. As shown in Table 1, in the proton conductor (A_(a)BO_(3−δ)),the A element was calcium (Ca), with an a value of 0.987. The oxygenamount was 2.972, and it was found that the charge of the compositionalformula was deviated from electrical neutrality by 0.03 toward thepositive side. As shown in Table 1, the proton conductivity at 100° C.was 0.02 S/cm, and the proton conductivity at 500° C. was 0.18 S/cm. Theactivation energy of proton conduction was determined to be 0.055 eV.

Example 12

An experiment was conducted similarly to Example 1 except that a filmwas formed by using a sintered target having an element ratio ofCa:Zr:Y=10:9:1. Table 1 shows the structure, mole fractions, and protonconductivity of the resultant film of proton conductor.

It was confirmed that the proton conductor had a perovskite-type crystalstructure. As shown in Table 1, in the proton conductor(A_(a)B_(1−x)B′_(x)O_(3−δ)), the A element was calcium (Ca), with an avalue of 0.95. The B element was zirconium (Zr); and the B′ element wasyttrium (Y), with an x value of 0.099. Moreover, the oxygen amount was2.926, and it was found that the charge of the compositional formula wasdeviated from electrical neutrality by 0.051 toward the negative side.As shown in Table 1, the proton conductivity at 100° C. was 0.030 S/cm,and the proton conductivity at 500° C. was 0.21 S/cm. The activationenergy of proton conduction was determined to be 0.055 eV.

Example 13

An experiment was conducted similarly to Example 1 except that a filmwas formed by using a sintered target having an element ratio ofBa:Sr:Zr:Y=5:5:8:2. Table 1 shows the structure, mole fractions, andproton conductivity of the resultant film of proton conductor.

It was confirmed that the proton conductor had a perovskite-type crystalstructure. As shown in Table 1, in the proton conductor(A_(a)B_(1−x)B′_(x)O_(3−δ)), the A element was barium (Ba) and strontium(Sr), with the Ba fraction being 0.39 and the Sr fraction being 0.43,with an a value of 0.92. The B element was zirconium (Zr); and the B′element was yttrium (Y), with an x value of 0.192. Moreover, the oxygenamount was 2.876, and it was found that the charge of the compositionalformula was deviated from electrical neutrality by 0.104 toward thenegative side. As shown in Table 1, the proton conductivity at 100° C.was 0.290 S/cm, and the proton conductivity at 500° C. was 0.41 S/cm.The activation energy of proton conduction was determined to be 0.021eV.

Comparative Example 1

An experiment was conducted similarly to Example 1 except that a filmwas formed by using a sintered target having an element ratio ofBa:Zr:Y=6:7:3. Table 1 shows the structure, mole fractions, and protonconductivity of the resultant film of proton conductor.

It was confirmed that the proton conductor had a perovskite-type crystalstructure. As shown in Table 1, in the proton conductor(A_(a)B_(1−x)B′_(x)O_(3−δ)), the A element was barium (Ba), with an avalue of 0.603. The B element was zirconium (Zr); and the B′ element wasyttrium (Y), with an x value of 0.315. Moreover, the oxygen amount was2.445, and it was found that the charge of the compositional formula waselectrically neutral. As shown in Table 1, the proton conductivity at100° C. was 3.3×10⁻⁶ S/cm, and the proton conductivity at 500° C. was8.4×10⁻³ S/cm. The activation energy of proton conduction was determinedto be 0.454 eV.

Comparative Example 2

An experiment was conducted similarly to Example 1 except that a filmwas formed by using a sintered target having an element ratio ofBa:Zr:Y=7:7:3. Table 1 shows the structure, mole fractions, and protonconductivity of the resultant film of proton conductor.

It was confirmed that the proton conductor had a perovskite-type crystalstructure. As shown in Table 1, in the proton conductor(A_(a)B_(1−x)B′_(x)O_(3−δ)), the A element was barium (Ba), with an avalue of 0.635. The B element was zirconium (Zr); and the B′ element wasyttrium (Y), with an x value of 0.318. Moreover, the oxygen amount was2.55, and it was found that the charge of the compositional formula wasdeviated from electrical neutrality by 0.148 toward the negative side.As shown in Table 1, the proton conductivity at 100° C. was 6.6×10⁻⁶S/cm, and the proton conductivity at 500° C. was 9.5×10⁻³ S/cm. Theactivation energy of proton conduction was determined to be 0.419 eV.

Comparative Example 3

An experiment was conducted similarly to Example 1 except that a filmwas formed by using a sintered target having an element ratio ofBa:Zr:Y=10:9:1. Table 1 shows the structure, mole fractions, and protonconductivity of the resultant film of proton conductor.

It was confirmed that the proton conductor had a perovskite-type crystalstructure. As shown in Table 1, in the proton conductor(A_(a)B_(1−x)B′_(x)O_(3−δ)), the A element was barium (Ba), with an avalue of 1.00. The B element was zirconium (Zr); and the B′ element wasyttrium (Y), with an x value of 0.101. Moreover, the oxygen amount was3.051, and it was found that the charge of the compositional formula wasdeviated from electrical neutrality by 0.203 toward the negative side.As shown in Table 1, the proton conductivity at 100° C. was 4.2×10⁻⁶S/cm, and the proton conductivity at 500° C. was 8.5×10⁻³ S/cm. Theactivation energy of proton conduction was determined to be 0.436 eV.

TABLE 1 mole mole oxygen A fraction B B′ fraction amount chargeconductivity (S/cm) activation sample element a element element x 3-δdeviation 100° C. 500° C. energy (eV) Example 1 Ba 0.904 Zr — 0 2.8760.056 0.12 0.25 0.041 Example 2 Ba 0.975 Zr•Ce Nd 0.099 2.975 −0.0990.33 0.48 0.022 Example 3 Ba 0.539 Zr Y 0.48 2.363 −0.128 0.39 0.710.035 Example 4 Ba 0.522 Zr•Ti In 0.22 2.474 −0.124 0.34 0.67 0.037Example 5 Ba 0.966 Zr Y 0.197 2.932 −0.129 0.37 0.78 0.039 Example 6 Sr0.980 Zr — 0 2.969 0.022 0.10 0.21 0.055 Example 7 Sr 0.965 Zr Yb 0.052.955 −0.03 0.10 0.22 0.055 Example 8 Sr 0.736 Zr•Ce Y 0.27 2.616 −0.030.13 0.25 0.042 Example 9 Sr 0.686 Zr In 0.351 2.542 −0.063 0.13 0.240.041 Example 10 Sr 0.563 Zr Y 0.474 2.358 −0.063 0.18 0.30 0.025Example 11 Ca 0.987 Zr Y 0 2.972 0.03 0.02 0.18 0.055 Example 12 Ca 0.95Zr Y 0.099 2.926 −0.051 0.030 0.21 0.055 Example 13 Ba•Sr 0.92 Zr Y0.192 2.876 −0.104 0.29 0.41 0.021 Comparative Ba 0.603 Zr Y 0.315 2.4450.00 3.3 × 10⁻⁶ 8.4 × 10⁻³ 0.454 Example 1 Comparative Ba 0.635 Zr Y0.318 2.55 −0.148 6.6 × 10⁻⁶ 9.5 × 10⁻³ 0.419 Example 2 Comparative Ba1.00 Zr Y 0.101 3.051 −0.203 4.2 × 10⁻⁶ 8.5 × 10⁻³ 0.436 Example 3

As shown in Table 1, in the proton conductors of Examples 1 to 13, themole fractions a and x when assuming a sum of B and B′ to be 1 satisfy0.5<a≦1.0 and 0.0≦x≦0.5. Moreover, in the proton conductors of Examples1 to 13, the charge of the compositional formula of the elementscomposing the perovskite structure is deviated from electricalneutrality. Specifically, the charge deviation is within the range from−0.128 to 0.057, and is non-zero. In the case where the B′ element isnot contained, the charge of the compositional formula is deviated fromelectrical neutrality in a range of more than 0 but 0.057 or less towardthe positive side. In the case where the B′ element is contained, thecharge of the compositional formula is deviated from electricalneutrality in a range of −0.129 or more but less than 0 toward thenegative side.

On the other hand, in the proton conductors of Comparative Examples 1 to3, the mole fractions a and x are within the range of Examples 1 to 13described above, but the charge of the compositional formula of theelements composing the perovskite structure is electrically neutral(Comparative Example 1), or deviated further toward the negative sidefrom −0.128.

As shown in Table 1, at 100° C., the proton conductors of Examples 1 to13 have proton conductivities which are four or more digits higher thanthose of the proton conductors of Comparative Examples 1 to 3. Also at500° C., the proton conductors of Examples 1 to 13 have protonconductivities which are ten or more times higher. In other words, in atemperature region of not less than 100° C. and not more than 500° C.,the proton conductors of Examples 1 to 13 have high protonconductivities of 10⁻² S/cm or more.

In particular, the proton conductors of Examples 1 to 13 have higherproton conductivities especially at a low temperature of about 100° C.than those of the proton conductors of Comparative Examples. Therefore,Examples 1 to 13 have activation energies of proton conduction which areabout one digit smaller than the activation energies of ComparativeExamples, being lower than 0.1 eV. On the other hand, the protonconductors of Comparative Examples 1 to 3 have activation energieshigher than 0.4 eV. This presumably indicates that, even at temperaturesof 100° C. or less, the proton conductors of Examples 1 to 13 havehigher proton conductivity than conventionally, without allowing theirproton conductivity to drastically decrease.

Thus, it was found from these Examples that the proton conductor of thepresent embodiment has higher proton conductivity than conventionallybecause the mole fractions a and x and the charge of its compositionalformula are within the aforementioned ranges. It was found that aremarkable improvement in proton conductivity over the conventionallevel is observed in a temperature range from about 100° C. to about500° C.

A proton conductor according to the present disclosure is used indevices related to hydrogen energy, e.g., fuel cells, hydrogen sensors,water vapor electrolytic devices, and hydrogenation devices, in astructure where it is interposed between an anode electrode and acathode electrode, and so on.

Embodiments have been described above as an illustration of thetechnique of the present disclosure. The accompanying drawings and thedetailed description are provided for this purpose. Thus, elementsappearing in the accompanying drawings and the detailed descriptioninclude not only those that are essential to solving the technicalproblems set forth herein, but also those that are not essential tosolving the technical problems but are merely used to illustrate thetechnique disclosed herein. Therefore, those non-essential elementsshould not immediately be taken as being essential for the reason thatthey appear in the accompanying drawings and/or in the detaileddescription.

The embodiments above are for illustrating the technique disclosedherein, and various changes, replacements, additions, omissions, etc.,can be made without departing from the scope defined by the claims andequivalents thereto.

What is claimed is:
 1. A proton conductor having a perovskite-typecrystal structure expressed by the compositional formula AaB1−xB′xO3−δ,wherein, A is at least one selected from among group 2 elements; B is atleast one selected from among group 4 elements and Ce; B′ is a group 3element, a group 13 element, or a lanthanoid element; 0.5<a≦1.0, x=0,and 0.0≦δ<3; and charge of the compositional formula is deviated fromelectrical neutrality in a range of more than 0 but less than +0.14. 2.The proton conductor of claim 1, wherein, A is at least one selectedfrom the group consisting of Ba, Sr, and Ca; B is at least one selectedfrom the group consisting of Zr, Ce, and Ti; and B′ is one selected fromthe group consisting of Yb, Y, Nd, and In.
 3. The proton conductor ofclaim 1, wherein the values a, x and δ are determined based onquantitative measurements of elements composing the proton conductor. 4.The proton conductor of claim 1, wherein an activation energy of protonconduction in a temperature range of not less than 100° C. and not morethan 500° C. is 0.1 eV or less.